1. Field of the Invention
The present invention relates to an automatic white balance regulating device employed in a color video camera and a color electronic camera.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an architecture of a conventional automatic white balance regulating device provided with a color temperature sensor system, which is employed in a color video camera. Light 2 reflected by an object passes through a lens 100 to an image pickup element 1. The pickup element 1 converts the light 2 from the object into a video signal E and applies it to a luminance signal processing circuit 3 and a chroma signal processing circuit 4. The luminance signal processing circuit 3 produces a luminance signal F from the video signal E and applies it to an adder 5. The chroma signal processing circuit 4 divides the video signal E received from the pickup element 1 into a blue signal B.sub.1, a red signal R.sub.1 and a green signal G.sub.1. The green signal G.sub.1 is applied to an input (-) of each of subtractors 6, 7, and the red signal R.sub.1 and the blue signal B.sub.1 are applied to a red signal gain control circuit (referred to as "R gain control circuit" hereinafter) 8 and a blue signal gain control circuit (referred to as "B gain control signal" hereinafter) 9, respectively.
Meanwhile, a red sensor (referred to as "R sensor" hereinafter) 10 outputs a red signal R.sub.2 corresponding to a red color component in the incident light, a green sensor (referred to as "G sensor" hereinafter) 11 outputs a green signal G.sub.2 corresponding to a green color component in the incident light, and a blue sensor (referred to as "B sensor" hereinafter) 12 outputs a blue signal B.sub.2 corresponding to a blue color component in the incident light, respectively. A divider 13a logarithmically converts the red signal R.sub.2 and the green signal G.sub.2 and subtracts one from the other to output a white balance control signal H.sub.1a representing the ratio of these signals, and a divider 14a logarithmically converts the blue signal B.sub.2 and the green signal G.sub.2 and subtracts one from the other to output a white balance control signal H.sub.2a representing the ratio of these signals.
An R gain control circuit 8 receives the white balance control signal H.sub.1a and changes its gain in an amount corresponding to the white balance control signal H.sub.1a. The B gain control circuit 9 receives the white balance control signal H.sub.2a and changes its gain in an account corresponding to the white balance control signal H.sub.2a. For example, as the level of the red signal R.sub.1 gets higher, the gain of the R gain control circuit 8 becomes smaller, while as the level of the red signal R.sub.1 gets lower, the gain of the R gain control circuit 8 becomes larger. The variation in the gain keeps the ratio of the red, green and blue signals, R.sub.1, G.sub.1 and B.sub.1 constant. In this way, the regulation of white balance is performed.
The R gain control circuit 8 amplifies the red signal R.sub.1 and applies the resultant signal to an input (+) of the subtractor 6. The B gain control circuit 9 amplifies the blue signal B.sub.1 and applies the resultant signal to an input (+) of the subtractor 7. The subtractor 6 subtracts the green signal G.sub.1 from the amplified red signal R.sub.1 and applies the resultant color difference signal I.sub.1 to a modulator 15. The subtractor 7 subtracts the green signal G.sub.1 from the amplified blue signal B.sub.1 and applies the resultant color difference signal I.sub.2 to the modulator 15. The modulator 15 performs vertical two-phase modulation using the color difference signals I.sub.1, I.sub.2 and applies the resultant modulation signal J to the adder 5. A synchronization signal generator 16 generates a vertical synchronization signal K and applies it to the adder 5. The adder 5 adds the luminance signal F, the modulation signal J and the vertical synchronization signal K and outputs an NTSC output L.
FIG. 2 is a block diagram showing an architecture of a conventional automatic white balance regulating device provided with a pickup color signal processing system, which is employed in a color video camera. The automatic white balance regulating device shown in FIG. 2 is different from the device shown in FIG. 1 in that the output from the chroma signal processing circuit 4 is used to obtain the white balance control signals H.sub.1b, H.sub.2b. The green signal G.sub.1 and the red signal R.sub.1 outputted from the chroma signal processing circuit 4 are applied to a divider 13b, and the green signal G.sub.1 and the blue signal B.sub.1 outputted from the same are applied to a divider 14b, respectively. The divider 13b logarithmically converts the red signal R.sub.1 and the green signal G.sub.1 and subtracts one from the other to output a white balance control signal H.sub.1b representing the ratio of these signals, while the divider 14b logarithmically converts the blue signal B.sub.1 and the green signal G.sub.1 and subtracts one from the other to output a white balance control signal H.sub.2b representing the ratio of these signals. Other components are the same as those of the device shown in FIG. 1.
The R gain control circuit 8 changes its gain in response to the white balance control signal H.sub.1b, while the B gain control circuit 9 changes its gain in response to the white balance control signal H.sub.2b. By thus altering the gain of each of the R and B gain control circuits 8 and 9, the automatic white balance regulation circuit is performed as stated above. Other operations of this device are similar to those of the device shown in FIG. 1.
The conventional automatic white balance regulating device, structured as stated above, has the problems mentioned below.
First, the problems of the color video camera provided with a color temperature sensor system, which is shown in FIG. 1, will be discussed. With this type of color video camera, when a scene of the outdoors is shot indoors through a window during the day, the R sensor 10, the G sensor 11 and the B sensor 12 receive the incident light from a light source set indoors. Thus, the white balance regulation is performed based upon the color temperature of the indoor light source. However, the light inputted to the pickup element 1 is the light 2 reflected by an object which is outdoors. As a result, there is the problem that an error arises in the white balance regulation because of the difference between the color temperature of the outdoor light source (i.e., the sun light) and that of the indoor light source.
Now, the problem of the color video camera provided with the pickup color signal processing system shown in FIG. 2 will be discussed. In the case where a person in a red sweater standing on a green lawn, for example, is shot with this type of color video camera, moving a lens 100 by a motor M to zoom out causes a major part of the shot scene (90% of the scene, for example) to be green. In this case, the red of the sweater hardly appear on the scene. On the other hand, moving the lens 100 by the motor M to zoom in causes a major part of the shot scene to be red. Thus when most of the shot scene lies in a single color, the white balance control signals H.sub.1b, H.sub.2b excessively alter in level, and the white balance regulation is inappropriately performed. Essentially, the lawn and the person in the red sweater are irradiated with sun light, and the white balance regulation is performed based upon the color components of the sun light. As a result, there is the problem that an error arises in the white balance regulation, and the green lawn is shot in blueish green and the red sweater is shot in brownish red.
The problem which is caused in the case where a scene at sunset is shot with a color video camera provided with either the color temperature sensor system or the pickup color signal processing system will be discussed. At sunset the red signals R.sub.1, R.sub.2 considerably increase in level, while the blue signals B.sub.1, B.sub.2 considerably decrease in level. Because of this, the white balance regulation is so performed that the gain of the R gain control circuit 8 gets small and the gain of the B gain control circuit 9 gets large. Regulating in this way, the scene at sunset is shot by the video camera with a red component being restrained. Thus, there is the problem that the scene shot by the video camera, though shot at sunset, has no difference from ordinary scenes shot during the day.